1. Field of the Invention
The present invention generally relates to image forming apparatuses and, more particularly, to an image forming apparatus such a printer having a saddle-stitching and center-folding function of printed sheets.
2. Description of the Related Art
Image outputting apparatuses such as a copy machine or a printer have become multi-functioned, and many image-outputting apparatuses are equipped with a memory device that can store a large amount of image data. Additionally, with such a multi-functionalization of the image outputting apparatuses, a post-processing apparatus has been diversified in its function such as a stapling function to staple output print papers.
As one of such techniques, Japanese Laid-Open Patent Application No. 11-151837 suggests a print apparatus having a stapling function, which performs a printing operation while canceling the stapling function if a number of print papers exceeds a number of sheets which can be stapled. Additionally, Japanese Laid-Open Patent Application No. 11-189365 suggests an image process post-processing apparatus, which determines whether or not a stapling can be applied, and prohibit a stapling operation if it is determined that a stapling is not applicable, and ejects print papers in a stapling part onto an eject tray. Further, Japanese Laid-Open Patent Application No. 9-183558 suggests a sheet post-processing apparatus having the same function as mentioned above.
Furthermore, Japanese Laid-Open Patent Application No. 2000-118860 suggests a technique to perform a saddle stitch process, which staples central portions of print papers in a paper feeding direction in addition to an end-stitching process, which staples an end portion of print papers. In such a saddle stitching process, print papers are temporarily conveyed to a stacking tray so as to align the print papers for stapling. Then, if a number of print papers is within a limit number for saddle stitching, the saddle stitching process is performed. On the other hand, if the number of print papers exceeds the limit number for saddle stitching, the print papers are ejected from the stacking tray to an outside tray without performing the saddle stitching process at that time.
In a conventional image forming apparatus, which performs such a saddle-stitching operation, if a number of sheets of printed paper temporarily stacked on the stack tray exceeds a maximum saddle-stitching number, the printed papers stacked on the stack tray is ejected therefrom and, thereafter, subsequent printed papers are stacked on the stack tray. For this reason, it takes a time to determine whether or not the number of printed papers stacked on the stack tray exceeds the maximum saddle-stitching number in addition to a time to stack the printed papers on the stack tray. Thereby, a processing time of the saddle-stitching operation is long, which lowers a productivity of the image forming apparatus. Moreover, in order to temporarily stack the printed papers on the stack tray, when the saddle stitching operation is not performed, a motor, which is not required to operate when performing the saddle-stitching operation, is unnecessarily driven, which results in an unnecessary consumption of electric power. Furthermore, since the printed papers are not bound if the printed papers are ejected from the stack tray to the outside tray, the printed papers are offset from each other and it is difficult to achieve a sufficient alignment.